java Filter过滤器 学习总结

1. Filter:

1). Filter 是什么 ?

①. JavaWEB 的一个重要组件, 可以对发送到 Servlet 的请求进行拦截, 并对响应也进行拦截.

②. Filter 是实现了 Filter 接口的 Java 类.即( implements Filter)

③. Filter 需要在 web.xml 文件中进行配置和映射. <!-- 注册 Filter --><filter>,<!-- 映射 Filter --><filter-mapping>

2). 如何创建一个 Filter, 并把他跑起来

①. 创建一个 Filter 类: 实现 Filter 接口: public class HelloFilter implements Filter

②. 在 web.xml 文件中配置并映射该 Filter. 其中 url-pattern 指定该 Filter 可以拦截哪些资源, 即可以通过哪些 url 访问到该 Filter

<!-- 注册 Filter -->

<filter>

<filter-name>helloFilter</filter-name>

<filter-class>com.atguigu.javaweb.HelloFilter</filter-class>

</filter>

<!-- 映射 Filter -->

<filter-mapping>

<filter-name>helloFilter</filter-name>

<url-pattern>/test.jsp</url-pattern>

</filter-mapping>

3). Filter 相关的 API:

创建一个HelloFilter如下：HelloFilter.java

public class HelloFilter implements Filter {

@Override

public void init(FilterConfig filterConfig) throws ServletException {

System.out.println("init..");

}

@Override

public void doFilter(ServletRequest request, ServletResponse response,

FilterChain chain) throws IOException, ServletException {

System.out.println("1. Before HelloFilter's chain.doFilter ..."); //1

//и┼лл

chain.doFilter(request, response);

System.out.println("2. After HelloFilter's chain.doFilter ..."); //2

}

@Override

public void destroy() {

System.out.println("destroy...");

}

}

①. Filter 接口:

> public void init(FilterConfig filterConfig): 类似于 Servlet 的 init 方法. 在创建 Filter 对象(Filter 对象在 Servlet 容器加载当前 WEB 应用时即被创建)后,

立即被调用, 且只被调用一次. 该方法用于对当前的 Filter 进行初始化操作. Filter 实例是单例的.

* FilterConfig 类似于 ServletConfig

* 可以在 web.xml 文件中配置当前 Filter 的初始化参数. 配置方式也和 Servlet 类似。

<filter>

<filter-name>helloFilter</filter-name>

<filter-class>com.atguigu.javaweb.HelloFilter</filter-class>

<init-param>

<param-name>name</param-name>

<param-value>root</param-value>

</init-param>

</filter>

> public void doFilter(ServletRequest request, ServletResponse response,

FilterChain chain): 真正 Filter 的逻辑代码需要编写在该方法中. 每次拦截都会调用该方法.

* FilterChain: Filter 链. 多个 Filter 可以构成一个 Filter 链.

- doFilter(ServletRequest request, ServletResponse response): 把请求传给 Filter 链的下一个 Filter,

若当前 Filter 是 Filter 链的最后一个 Filter, 将把请求给到目标 Serlvet(或 JSP)

- 多个 Filter 拦截的顺序和 <filter-mapping> 配置的顺序有关, 靠前的先被调用.

> public void destroy(): 释放当前 Filter 所占用的资源的方法. 在 Filter 被销毁之前被调用, 且只被调用一次.

eg.用户登陆拦截

1.UserNameFilter.java 用户名拦截

public class UserNameFilter implements Filter {

/**

* Default constructor.

*/

public UserNameFilter() {

// TODO Auto-generated constructor stub

}

/**

* @see Filter#destroy()

*/

public void destroy() {

// TODO Auto-generated method stub

}

/**

* @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)

*/

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

String initUser = filterConfig.getInitParameter("username");

String username = request.getParameter("username");

if(!initUser.equals(username)){

request.setAttribute("message", "用户名不正确");

request.getRequestDispatcher("/login.jsp").forward(request, response);

return;

}

chain.doFilter(request, response);

}

private FilterConfig filterConfig;

public void init(FilterConfig fConfig) throws ServletException {

this.filterConfig = fConfig;

}

}

2.PasswordFilter.java用户密码拦截

PasswordFilter

public class PasswordFilter implements Filter {

/**

* Default constructor.

*/

public PasswordFilter() {

// TODO Auto-generated constructor stub

}

/**

* @see Filter#destroy()

*/

public void destroy() {

// TODO Auto-generated method stub

}

/**

* @see Filter#doFilter(ServletRequest, ServletResponse, FilterChain)

*/

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

String initPassword = filterConfig.getServletContext().getInitParameter("password");

String password = request.getParameter("password");

if(!initPassword.equals(password)){

request.setAttribute("message", "密码不正确");

request.getRequestDispatcher("/login.jsp").forward(request, response);

return;

}

chain.doFilter(request, response);

}

/**

* @see Filter#init(FilterConfig)

*/

private FilterConfig filterConfig;

public void init(FilterConfig fConfig) throws ServletException {

this.filterConfig = fConfig;

}

}

3.web.xml

<context-param>

<param-name>password</param-name>

<param-value>1234</param-value>

</context-param>

<filter>

<display-name>UserNameFilter</display-name>

<filter-name>UserNameFilter</filter-name>

<filter-class>com.atguigu.javaweb.UserNameFilter</filter-class>

<init-param>

<param-name>username</param-name>

<param-value>Tom</param-value>

</init-param>

</filter>

<filter-mapping>

<filter-name>UserNameFilter</filter-name>

<url-pattern>/hello.jsp</url-pattern>

</filter-mapping>

<filter>

<display-name>PasswordFilter</display-name>

<filter-name>PasswordFilter</filter-name>

<filter-class>com.atguigu.javaweb.PasswordFilter</filter-class>

</filter>

<filter-mapping>

<filter-name>PasswordFilter</filter-name>

<url-pattern>/hello.jsp</url-pattern>

</filter-mapping>

用户密码正确去hello.jsp。否则去login.jsp。

login.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

<font color="red">${message }</font>

<br><br>

<form action="hello.jsp" method="post">

username: <input type="text" name="username" value="${param.username }"/>

password: <input type="password" name="password"/>

<input type="submit" value="Submit"/>

</form>

</body>

</html>

下面hello.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8"%>

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

<html>

<head>

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

<title>Insert title here</title>

</head>

<body>

Hello: ${param.username }

</body>

</html>

////begin.../////接下来：自定义的 HttpFilter, 实现自 Filter 接口/////////////////////////////////////

public abstract class HttpFilter implements Filter {

/**

* 用于保存 FilterConfig 对象.

*/

private FilterConfig filterConfig;

/**

* 不建议子类直接覆盖. 若直接覆盖, 将可能会导致 filterConfig 成员变量初始化失败

*/

@Override

public void init(FilterConfig filterConfig) throws ServletException {

this.filterConfig = filterConfig;

init();

}

/**

* 供子类继承的初始化方法. 可以通过 getFilterConfig() 获取 FilterConfig 对象.

*/

protected void init() {}

/**

* 直接返回 init(ServletConfig) 的 FilterConfig 对象

*/

public FilterConfig getFilterConfig() {

return filterConfig;

}

/**

* 原生的 doFilter 方法, 在方法内部把 ServletRequest 和 ServletResponse

* 转为了 HttpServletRequest 和 HttpServletResponse, 并调用了

* doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)

*

* 若编写 Filter 的过滤方法不建议直接继承该方法. 而建议继承

* doFilter(HttpServletRequest request, HttpServletResponse response,

* FilterChain filterChain) 方法

*/

@Override

public void doFilter(ServletRequest req, ServletResponse resp,

FilterChain chain) throws IOException, ServletException {

HttpServletRequest request = (HttpServletRequest) req;

HttpServletResponse response = (HttpServletResponse) resp;

doFilter(request, response, chain);

}

/**

* 抽象方法, 为 Http 请求定制. 必须实现的方法.

* @param request

* @param response

* @param filterChain

* @throws IOException

* @throws ServletException

*/

public abstract void doFilter(HttpServletRequest request, HttpServletResponse response,

FilterChain filterChain) throws IOException, ServletException;

/**

* 空的 destroy 方法。

*/

@Override

public void destroy() {}

}

///end...///////////////////////////////

百度经骗：http://jingyan.baidu.com/article/4dc40848bc67d0c8d846f178.html

百度经骗：http://jingyan.baidu.com/article/851fbc37c0ff2d3e1e15ab46.html

4). <dispatcher> 元素: 指定过滤器所拦截的资源被 Servlet 容器调用的方式，

可以是REQUEST,INCLUDE,FORWARD和ERROR之一，默认REQUEST.

可以设置多个<dispatcher> 子元素用来指定 Filter 对资源的多种调用方式进行拦截

①. REQUEST：当用户直接访问页面时，Web容器将会调用过滤器。如果目标资源是通过RequestDispatcher的include()或forward()方法访问时，那么该过滤器就不会被调用。

通过 GET 或 POST 请求直接访问。

②. FORWARD：如果目标资源是通过RequestDispatcher的forward()方法访问时，那么该过滤器将被调用，除此之外，该过滤器不会被调用。

或 <jsp:forward page="/..." /> 或 通过 page 指令的 errorPage 转发页面. <%@ page errorPage="test.jsp" %>

②. INCLUDE：如果目标资源是通过RequestDispatcher的include()方法访问时，那么该过滤器将被调用。除此之外，该过滤器不会被调用。

或 <jsp:include file="/..." />

④. ERROR：如果目标资源是通过声明式异常处理机制调用时，那么该过滤器将被调用。除此之外，过滤器不会被调用。

在 web.xml 文件中通过 error-page 节点进行声明:

<error-page>

<exception-type>java.lang.ArithmeticException</exception-type>

<location>/test.jsp</location>

</error-page>

<filter-mapping>

<filter-name>secondFilter</filter-name>

<url-pattern>/test.jsp</url-pattern>

<dispatcher>REQUEST</dispatcher>

<dispatcher>FORWARD</dispatcher>

<dispatcher>INCLUDE</dispatcher>

<dispatcher>ERROR</dispatcher>

</filter-mapping>

百度经骗：http://jingyan.baidu.com/article/1876c85292dee4890a13767c.html
http://jingyan.baidu.com/article/ed2a5d1f5e33b209f7be1746.html
1、目录：login/a.jsp,b.jsp,c.jsp,d.jsp,e.jsp，

login.jsp登录页，

doLogin.jsp 登录页，

list.jsp 列出a,b,c,d

2.web.xml

<!-- 用户信息放入到 session 中的键的名字 -->

<context-param>

<param-name>userSessionKey</param-name>

<param-value>USERSESSIONKEY</param-value>

</context-param>

<!-- 若未登录, 需重定向的页面 -->

<context-param>

<param-name>rediretPage</param-name>

<param-value>/login/login.jsp</param-value>

</context-param>

<!-- 不需要拦截(或检查)的 URL 列表 -->

<context-param>

<param-name>uncheckedUrls</param-name>

<param-value>/login/a.jsp,/login/list.jsp,/login/login.jsp,/login/doLogin.jsp,/login/b.jsp</param-value>

</context-param>

<filter>

<filter-name>loginFilter</filter-name>

<filter-class>com.atguigu.javaweb.login.LoginFilter</filter-class>

</filter>

<filter-mapping>

<filter-name>loginFilter</filter-name>

<url-pattern>/login/*</url-pattern>

</filter-mapping>

3.LoginFilter.java

public class LoginFilter extends HttpFilter{

//1. 从 web.xml 文件中获取 sessionKey, redirectUrl, uncheckedUrls

private String sessionKey;

private String redirectUrl;

private String unchekcedUrls;

@Override

protected void init() {

ServletContext servletContext = getFilterConfig().getServletContext();

sessionKey = servletContext.getInitParameter("userSessionKey");

redirectUrl = servletContext.getInitParameter("rediretPage");

///login/a.jsp,/login/list.jsp,/login/login.jsp,/login/doLogin.jsp

unchekcedUrls = servletContext.getInitParameter("uncheckedUrls");

}

public void doFilter(HttpServletRequest request,

HttpServletResponse response, FilterChain filterChain)

throws IOException, ServletException {

//1. 获取请求的 servletPath

// /login/b.jsp

String servletPath = request.getServletPath();

//2. 检查 1 获取的 servletPath 是否为不需要检查的 URL 中的一个, 若是, 则直接放行. 方法结束

List<String> urls = Arrays.asList(unchekcedUrls.split(","));

if(urls.contains(servletPath)){

filterChain.doFilter(request, response);

return;

}

//3. 从 session 中获取 sessionKey 对应的值, 若值不存在, 则重定向到 redirectUrl

Object user = request.getSession().getAttribute(sessionKey);

if(user == null){

response.sendRedirect(request.getContextPath() + redirectUrl);

return;

}

//4. 若存在, 则放行, 允许访问.

filterChain.doFilter(request, response);

}

}